Telecentric optical systems are often employed in a measuring projector. The measuring projector allows a measurement of various values by illuminating the profile of an object to be measured, projecting the profile onto a screen, enlarging the projected image by a projection optical system and comparing the projection with a reference figure on the screen. For such systems, it is preferable that the projection be telecentric. That is, a principal ray passes through the optical system parallel with the optical axis.
However, the need exists for a telecentric zoom lens system having a reduced level of distortion in low tolerance applications. A further need exists for a telecentric zoom lens that has inherent color correction and reduced aberrations. A need also exists for a telecentric zoom lens that provides a constant speed on an image side of the lens. The need also exists for a method and apparatus of constructing such a telecentric zoom lens.